A gas diffusion electrode equipped ion exchange membrane electrolyzer provided with a gas diffusion electrode is utilized as a means for reducing electrolysis voltage by causing a reaction with a gas introduced from outside at the gas diffusion electrode.
In a gas diffusion electrode equipped ion exchange membrane electrolyzer for alkali metal chloride aqueous solution wherein the gas diffusion electrode is used as a cathode, an alkali chloride aqueous solution is supplied to an anode chamber so as to generate a chlorine gas at an anode. On the other hand, an oxygen-containing gas is supplied to a cathode chamber, whereby at the gas diffusion electrode, the oxygen is reduced, and further, an alkali metal hydroxide aqueous solution is generated.
When operation of the electrolyzer is stopped, a chlorine evolution reaction and an oxygen reduction reaction are stopped; while the potentials of the anode and anode chamber are kept at a chlorine evolution potential since the chlorine exists in solution in the alkali metal chloride aqueous solution which is an anolyte. On the other hand, the gas diffusion electrode and cathode chamber are subjected to a condition where they contact the alkali metal hydroxide aqueous solution and oxygen-containing gas, so that the voltage potentials of the gas diffusion electrode and cathode gas chamber are kept at an oxygen reduction potential.
However, when the operation is stopped, generation of the alkali metal hydroxide aqueous solution is stopped in the cathode chamber although the anolyte remains in the anode chamber, so that only a tiny amount of alkali metal hydroxide aqueous solution retained in a hydrophilic layer exists in the cathode chamber side.
When the anolyte in the anode chamber is transferred through the ion exchange membrane and poured into the cathode chamber according to the concentration gradient between the anode chamber and cathode chamber, a catholyte is replaced by the anolyte.
Originally, the cathode chamber is made of a material having a sufficient corrosion resistance against the alkali metal hydroxide aqueous solution having alkaline property. However, the corrosion resistance of the cathode chamber is not sufficient against, e.g., the alkali metal chloride aqueous solution having a pH ranging from acidic to neutral.
There is proposed, as an electrolyzer protection method which is employed in a gas diffusion electrode equipped ion exchange membrane electrolyzer in which a cathode chamber and an anode chamber are separated by an ion exchange membrane and which prevents corrosion of a cathode chamber and degradation of a catalyst during the stop time of the electrolyzer, a method of protecting the gas diffusion electrode equipped ion exchange membrane electrolyzer by stopping supply of an oxygen-containing gas to the cathode chamber and replacing the oxygen-containing gas atmosphere in the cathode chamber with an alkali metal hydroxide aqueous solution (refer to e.g., Patent Document 1).